1. Field of the Invention
This invention relates to a low noise telephone line interface for data access arrangements (DAA). More particularly, it relates to a line powered DAA having controllable DC termination and AC modulation.
2. Background of Related Art
Many portable computer devices utilize modems and other data devices for communicating over a telephone line. In such devices, battery size and weight is an important consideration. A balance must be reached between the size and weight of the overall device, which is dictated largely by the choice of battery, and an acceptable length of operation between charges.
Unfortunately, although operating an acceptable length of time when running typical application programs, the modems and other data communication devices of a portable computer utilize a large amount of power when communicating over a telephone line. The battery source which powers both the portable computer device and its modem is typically sized for general computing applications, and runs out of power quickly when actively communicating over a telephone line through a modem. Portable computer devices such as personal digital assistants (PDAs), hand held PCs (HPC), PCMCIA modems, and portable data terminals are designed to operate up to several hours on a single battery charge, but operate only fractions of an hour on a single battery charge when communicating via modem. Thus, although portable computer devices operate a sufficient length of time for quick data transfers over a modem powered completely by a battery, they typically require that external AC power be applied to allow for longer uses of the modem. It is therefore desirable for battery powered computer devices including a modem to draw power, in addition to the inherent battery, from a secondary power source.
The DC power inherent in a telephone line provides a convenient source of power, but there are often limitations and restrictions which limit the ability of a modem to derive power from the telephone line. For instance, present regulations in the United States are such that significant current may only be drawn from the telephone line when the telephone or modem is in an off-hook or active condition. In order to hold the telephone line in an off-hook condition, current in the approximate range of 20 milliamps (mA) to 150 mA must be drawn. Thus, the maximum amount of current drawn from the telephone line is limited.
Modems which are designed to be powered entirely from the telephone line are known, but these designs either suffer from an extremely constrained power budget, or are wasteful of the available current. Moreover, modems in general are also subject to government constraints, e.g., FCC Part 68 requirements for telephones in the U.S., and limitations on effects and noise which may be placed back on the telephone line, placing further restrictions on the use of power from the telephone line.
A Data Access Arrangement (DAA) provides the physical interface between a data source such as a modem, and a telephone line. The DAA is responsible for presenting the proper DC termination and AC modulation characteristics to the telephone line. For instance, the DAA must draw a minimum amount of DC current when in the off-hook condition to hold the telephone line in an off-hook condition, but at the same time must draw no more than a maximum amount of current while in the off-hook condition. Thus, the DC termination or load must be within prescribed limits corresponding to the pertinent telephone standards of the country in which the DAA is being used. Accordingly, the modem must operate with no more than the prescribed maximum current available from a telephone line in the particular country in which the DAA is being used.
In today""s global economy, it is desirable to design and manufacture products for use in any of a plurality of different countries, not just for a single country. However, the regulations with respect to the available amount of power on a telephone line in the different countries vary greatly, as does the minimum amount of current the customer premises equipment must draw (i.e., the DC load) to guarantee an off-hook condition. Thus, a DAA used in one country must provide DC termination and AC modulation characteristics to a telephone line which may be and often are entirely different from the DC termination and AC modulation characteristics presented by a DAA used in another country. Accordingly, manufacturers conventionally manufacture different DAAs including different physical components for each of the varying countries.
FIG. 1 shows a typical DAA including a parallel combination of an AC line modulator 102 and a DC termination circuit 103. A modem or other data device including the DAA shown in FIG. 1 provides an AC modulation signal 108 to drive the AC line modulator 102. When used in countries with differing standards, many of the components in the DC termination circuitry 103 and/or in the AC line modulator 102 are changed to conform to the telephone company standards in the particular country in which the DAA is being used. This unfortunately requires products incorporating the DAAs to be manufactured and packaged separately for each of a plurality of varying countries.
There is a need for a DAA which provides low noise line power for a modem or other data device, and which is software customizable for each of a plurality of varying countries without requiring different hardware components to be in conformance with the different DC termination and AC modulation requirements of those particular countries.
A line powered telephone line interface or data access arrangement (DAA) according to the principles of the present invention comprises a shunt regulator to draw power from the telephone line. A line modulator is placed in series with the shunt regulator, with a sense resistor in series therebetween. To reduce noise in the DAA, the DAA includes a single ended feedback path from one terminal of the sense resistor to the line modulator.
A method in accordance with the principles of the present invention comprises drawing power from a telephone line with a shunt regulator. The telephone is modulated with a line modulator in series with the shunt regulator. A level of noise in the telephone line interface is sensed and fed back to the line modulator.